"It means there'll be a lot less bright stars because at the end of their lives these stars become very bright giant stars. Seventy per cent of those stars will be missing so there'll be less lights coming from that cluster."

The Milky Way galaxy contains about 150 large tightly packed spherical balls, known as globular clusters, each containing about a million stars.

"Clusters are used by astronomers to test theories on stellar evolution," says Campbell.

The life cycle of a star begins with it shining by fusing hydrogen in its core into helium. The Sun is currently going through this fusion phase.

Once it runs out of hydrogen in its core, it expands to become a bigger and cooler star called a red giant, during which time it burns helium in its core.

When stars like the Sun run out of helium, they move on to their final phase of life, which astronomers call the asymptotic giant branch, or AGB phase.

During this final phase, stars alternate between burning shells of hydrogen and helium around the core, a process that ejects material in the form of gas and dust, which helps create future generations of stars, planets and life.

"And that's where things seem to be going wrong with stars in this cluster, most of them aren't getting up to that final phase," says Campbell.

Not following the rules

Campbell and colleagues looked at stars in globular cluster NGC 6752, in the southern constellation Pavo which is about 13,000 light-years away in the halo of the Milky Way.

By studying the light spectrum of these stars, scientists could measure how much sodium was in each red giant and AGB star.

"If you look at the red giants ... 70 per cent are sodium rich and 30 per cent sodium poor," says Campbell.

But when astronomers analysed the AGB stars, they found them all to be sodium poor.

"Something has happened to the sodium rich stars," says Campbell.

The results were a surprise for Campbell and colleagues.

"We don't think sodium affects their evolution, but in our studies it seems to be a good tracer of which stars will not make it to the final stage of life," says Campbell.

He says what is causing them not to reach this stage is still a mystery.

"We need to put that into the stellar model, so we can simulate what's going on in reality, the models need to be updated."

The team expects similar results will be found for other clusters, and further observations are planned.